A waveform generally refers to a shape of a signal, such as a wave, that is moving in a medium (for example, a solid, liquid or gaseous medium). In cases wherein a direct visual image of a shape of a propagated signal cannot be obtained, a waveform, which in these cases may also be referred to as a moving waveform, describes a shape of a graph which is varying over time or distance. In other words a moving waveform shows a series of propagating waves, each representing a different time or distance.
In capnography for example, a capnograph collects samples of a patient's breath, senses and calculates the real time CO2 concentration (as partial CO2 pressure) of the sample. The calculated CO2 concentration over time is depicted on an appropriate display as a moving waveform. The resolution of the moving waveform and the sweep time are such that a user can identify breath cycles on the display. The information obtained in capnography may be used to determine a condition of a patient.
In the case of capnography, the “x” axis of the displayed moving waveform is time and is generally defined for enough time so that at least 2 to 3 wave cycles can be captured. Since different age groups have different breath cycle times, the length of the “x” axis may often be changed as a function of the age. For example, since neonates breathe at relatively fast rates, normally 50 to 80 Breaths per Minute (BPM), the time required to capture at least 2 to 3 wave cycles will be shorter than the time required to capture the same number of wave cycles in adults.
The “y” axis of the displayed moving waveform is defined either in units of partial pressure (mmHg, kpa or other units) or in volume percent (Vol. %). The height of the “y” axis is generally defined such that a standard breath concentration (37 mmHg) will reach close to approximately ⅔ of the axis (for example 50 mmHg). When high CO2 concentrations are realized, this may be increased (for example to 100 mmHg).
In order for a waveform to be at least minimally representative of the changes in CO2 concentration over time in a breath cycle, a reading (or a point) should be taken at least once in every 200 millisecond (msec) for standard adult breath rates, though every 50 msec may be preferred. With high respiration rates up to 150 BPM, it may be imperative to read every 50 msec. The more data points (resolution), the more a user or an operator (such as a physician or a nurse) can observe characteristic shapes in the waveform.
The waveform generally includes both clinically relevant characteristics but also clinically insignificant characteristics on the waveform. In general, clinically irrelevant (insignificant) effects on the waveform originate from external stimuli such as talking, movement, coughing, eating or the like, whereas a clinically significant effect on the waveform may originate from the patient physiological condition, for example, respiratory and/or cardiac condition. Such conditions may include asthma, congestive heart failure (CHF), chronic obstructive pulmonary disease (COPD), sedation or treatment with drugs, such as in pain management or any other medical condition.
In addition to the potential clinical value and relevancy related to the characteristics of each individual waveform, the patterns and characteristics of consecutive waveforms or groups of waveforms and their changes, rates of change and other characteristics may also be relevant. An example of characteristics of consecutive waveforms may be seen in Cheyne Stokes breathing which can be diagnosed only while analyzing consecutive waveforms as opposed to one or two individual waveforms.
There may also be relevancy to a periodicity and repeatability of the waveforms on one hand, and to an erratic behavior and dispersion of the characteristics on the other hand. Sometimes, the clinical meaning of the above may be different for an awaken patient and a patient who is asleep (sedated or unconscious). With intubated patients, the waveforms are generally more repeatable, especially for duration and duty cycle of the breath cycle because of the ventilator settings and generally sedated state of the patient. With non-intubated patients the shapes of each waveform are more sporadic.
Several problems are associated with the displaying and usability of prior art moving waveforms, such as the CO2 concentration moving waveforms produced and presented in capnography. These problems include for example the following:
1. The existing Capnographs show a series of two or three waveforms at a time and they are continuously being built up and crossing the screen relative to the patient's breath cycle, with no synchronization to the axis. In other words, the waveform may sometimes start in the middle of a breath cycle at “x” (time)=0, or may start towards the end of breath cycle, depending on how the respiration rate (RR) fits the time span given for the “x” axis displayed. This moving waveform makes it difficult to focus on characteristics and changes of the waveform which may be of interest.2. A user (such as a physician or a nurse) can only compare between two or three waves at any given time, hence patterns over groups and sets of waveforms are difficult and sometimes impossible to notice.3. There are many artifacts that can cause changes to each waveform which makes it difficult to distinguish between patterns that have physiological/clinical importance and patterns which are only artifacts.4. In addition to the many possible shapes of the waveform, there are also the scale factors of the waveform. The scale factors include heights, widths, duty cycles and the like. In spontaneous breathing patients particularly, these scale factors become an infinite number of possible combinations, changing on the screen at all the time, many of them are not yet understood even by respiratory experts.5. Characteristics of the waveform that may have physiological/clinical significance are often slight and not easily recognized, for example, for some of the reasons mentioned above (even if the user is well versed in these patterns). 6. It is difficult to differentiate between dominant, recurring patterns and those that are erratic, non-recurring patterns.7. Although, in addition to real time sweeping of the moving waveform, a capnograph for example, may also provide trend data (for example, of the End-Tidal Carbon Dioxide, EtCO2) of the waveforms, and sometimes an ability to return to a waveform from the past related to an event or requested reference, this requires large memory banks which would require a very strong computer with large memory to save even a few hours of previous waveforms.8. It is difficult to notice a change in certain characteristics of a waveform (for example, slopes or other characteristics) without the ability to compare with a reference and/or a baseline.
These problems and others often create a feeling of desperation by nurses and doctors regarding the usability of these waveforms besides receiving a visual sign that there is a breathing cycle occurring.
There is thus a need in the art for methods and apparatuses that would produce usable waveforms.